An increasing number of cell types, including peripheral blood mononuclear cells (PBMCs), have been demonstrated to release heat shock proteins (Hsps). In this paper we investigate further the hypothesis that Hsps are danger signals. PBMCs and Jurkat cells released Hsp70 (0.22 and 0.7 ng/10(6) cells, respectively) into medium over 24 h at 37 degrees C. Release of Hsp70 was stimulated 10-fold by GroEL (P < 0.001) and more than threefold by lipopolysaccharide (LPS) (P < 0.001). Although Hsp60 could be detected in the medium of cells cultured at 37 degrees C for 24 h, the low rates of release were due probably to cell damage. Significant release of Hsp60 was observed when Jurkat cells were exposed to GroEL (2.88 ng/10(6) cells) or LPS (1.40 ng/10(6) cells). The data are consistent with the hypothesis that Hsp70 and Hsp60 are part of a danger signalling cascade in response to bacterial infection.